This invention relates to a method for making a heat-insulated structural section assembly for use in a window and door frames, consisting of two structural metal sections transversely connected to each other by a pair of parallel, plastic heat-insulating bars spaced from each other and having longitudinal ends of swallow-tail cross-sectional shape which are successively received between pairs of longitudinally extending opposite grooves each defined by upper and lower flanges in said structural sections, whereby said pair of heat-insulating bars define an enclosed space, in which method those grooves loosely receiving first one of said heat-insulating bars are closed upon said one heat-insulating bar by continuously pressing said upper flanges of the respective grooves against said one heat-insulating bar.